1. Field of the Invention
The present invention relates to a variable structure circuit topology for high intensity discharge (HID) lamp electronic ballasts.
2. Description of Background Information
In electronic high intensity discharge lamp ballasts, there are two distinctly different methods to drive the lamp. The first method is to drive the lamp with high frequency sinusoidal current, and the second is to drive the lamp with low frequency rectangular current. The high frequency sinusoidal current method tends to give rise to acoustic resonance. Accordingly, low frequency rectangular current wave operation remains the favored technique for electronic high intensity discharge lamp ballasts because of the acoustic resonance problem associated with the high frequency method.
Two fundamental approaches are generally taken to generate a low frequency (less than 1 kHz) rectangular current with very small high frequency ripple to the lamp, as shown in comparative examples in FIGS. 1a and 1b. FIG. 1a shows a buck power regulator with pulse ignition in discontinuous inductor current mode, including switching element, inductor, diode and capacitor. In this case, the current in the inductor has very large triangular high frequency ripple.
U.S. Pat. No. 5,428,268 to Melis et al., issued Jun. 27, 1995, describes one implementation substantially corresponding to the example of FIG. 1a. As shown in FIG. 5A of Mehlis et al., the average part of the inductor current goes to the lamp, while the AC part of the inductor current is filtered by a capacitor C20 across the lamp. The patent to Melis et al. includes no specific mention of the actual values of capacitance C20 and inductance L20. However, to sufficiently filter the AC high frequency current to be below an acceptable level, and to maintain discontinuous mode operation for switching efficiency, the capacitance C20 has to be very large and the inductance L20 has to be quite small. The characteristic impedance of the circuit is low because of the large value of capacitance C20 and small value of inductance L20. It is known that the resonant voltage can be approximated by the characteristic impedance multiplied by the resonant current. Accordingly, generation of a high ignition voltage using the resonant method necessarily suffers from high circulating resonant current in the resonant elements and driving source switches. For example, when C is equal to 0.47 xcexcF, L is equal to 890 xcexcH, and Vp is equal to 3 kVpeak, the resonant current will be 69 Apeak. Obviously, the pulse method as disclosed in Melis et al. is the only logical method to ignite the lamp for the circuit arrangement and for the mode of operation disclosed therein.
The disadvantages of pulse mode ignition are clearly explained in commonly assigned U. S. Pat. No. 5,932,976. FIG. 1b of the present application shows a comparative example of a buck power regulator with high frequency resonant ignition, similar to that of U.S. Pat. No. 5,932,976, and with continuous inductor current mode. Shown in FIG. 1b are switches Q1, Q2; diodes D1-D4, inductor L1, and capacitors C1, Ca and Cb. In this case, the current in the inductor L1 has a very small triangular high frequency ripple superimposed on the low frequency rectangular current. Both the average part of the inductor current and the AC part of the inductor current flow through the lamp LMP. The parallel capacitor C1 with small capacitance is present only for the purpose of generating ignition voltage, and the burden of filtering the high frequency ripple is almost entirely on the inductor L1. The disadvantages of this arrangement become apparent when it is considered that the high frequency attenuation is only xe2x88x9220 dB/decade (logarithmic decade) for frequencies above the corner frequency (the corner frequency being formed by the lamp LMP impedance and the inductance L1). To achieve ripples low enough to avoid any acoustic resonance problems, the physical size of the inductor L1, and the inductance itself, must be fairly large. A side effect of large inductance is an increased glow-to-arc transition time. Another disadvantage of this arrangement is that the switching elements Q1, Q2 are in hard switching mode during the switch turn-on interval. The necessary switches are expensive because external ultra-fast freewheeling diodes in the order of 20-50 nS reverse recovery time are required. Moreover, switching losses are relatively high.
U.S. Pat. No. 4,904,907 to Allison et al., issued Feb. 27, 1990, discloses a modification of the continuous mode operation discussed above, in which (as shown in FIG. 5 of Allison et al.) an LC parallel resonant network (part of T301 and C304, C305 combination) is inserted into the buck inductor (part of T301). The inserted LC parallel resonant network has a resonant frequency at the buck operating frequency, and the fundamental frequency of the buck power regulator is attenuated significantly. A drawback of the circuit of Allison et al. is that the attenuation factor is highly sensitive to the frequency variation of the buck converter.
For example, the impedance of an LC parallel network can be calculated as:                                           Z            p                    ⁡                      (            w            )                          =                  "LeftBracketingBar"                      wLp                          {                              1                -                                                      (                                          w                      wp                                        )                                    2                                            }                                "RightBracketingBar"                                    (        1        )            
where wp is the parallel LC resonant frequency. The impedance at 1% and 3% deviations from the resonant frequency is Zp=(1.01 wp)=50.2 and Zp=(1.03wp)=16.9, respectively. It can been seen that a 2-percentage point variation in the operating frequency will cause the attenuation impedance to vary by a factor of 3, which in turn will cause the high frequency ripple to be attenuated by almost the same factor.
In the above mentioned two patent disclosures (U.S. Pat. Nos. 5,428,268 and 4,904,907), two stages of conversion are required to regulate the power and to supply a rectangular current to the lamp. The first stage regulates the lamp power and limits the current in the lamp during warm-up phase. The high frequency ripple is also attenuated by the filters in the first stage. The second stage is a fall bridge inverter that takes the DC output from the buck regulator and converts the DC output to a low frequency rectangular current (AC) for the lamp. A pulse ignition circuit is invariably required to ignite the lamp.
U.S. Pat. No. 4,912,374 to Nagase et al., issued Mar. 27, 1990, discloses a high frequency resonant ignition technique, although such is not the primary subject matter of this patent and is not specifically mentioned therein. In this topology, e.g., FIGS. 1 and 3 of Nagase et al., the power control stage and the inverter stage are combined in a half bridge/full bridge topology. The power control stage is combined with the output inverter, and in order to prevent acoustic resonance, the output inductor L1 and the capacitor C1 across the lamp must provide sufficient filtering to keep the high frequency component of the lamp current to a minimum. Consequently, the capacitance C1 is large, in the order of 1/10 micro-farads (xcexcF). When this arrangement is operated at a high frequency and the lamp is OFF, the resonant circuit formed by the inductor and capacitor produces a high voltage to ignite the lamp. Very large circulating current flows in the circuit because of the large capacitance value and the relatively smaller inductance value. When the lamp is in high frequency operation, high frequency current is produced in the lamp. During the low frequency mode, the switching pattern is changed to one that would control the lamp power and limit the lamp current. Fundamentally, the disclosure of Nagase et al. has the same disadvantages as the comparative example of FIG. 1a of the present disclosure in discontinuous mode operation, except that resonant ignition is implied.
U.S. Pat. No. 6,020,691 to Sun et al., issued Feb. 1, 2000, discloses a driving circuit for high intensity discharge lamp electronic ballast. FIG. 2 illustrates a schematic diagram of the Sun circuit that addresses some of the problems associated with the demand for high circulating current for resonant ignition, low efficiency if operated in continuous mode, ripple sensitivity to the operating frequency, and the need for pulse ignition.
The design for the first stage L-C filter is intricately coupled to the second stage L-C filter. One is forced to choose the first stage LC filter resonant frequency to be much lower than the second stage LC filter resonant frequency. This implies larger circuit componentsxe2x80x94increasing cost, size, and weight. On the other hand, zero current switching (ZCS) for higher circuit efficiency is achieved passively. That is, ZCS is possible because of the choice of the inductor L1 and the operating frequency rather than active switching of Q1 and Q2. This places a restriction on the choice of these two important circuit parameters. Another significant problem is the position of the circuit ground. In order to accurately sense the lamp circuit, the ground is chosen as the center point of the two bus capacitors. Consequently, the input power factor correction circuit ground and the output stage ground are at different potentials. Hence, level shifting circuits, and opto-isolators, become necessary, thus making the device more expensive and less reliable. In addition, as average lamp current is being sensed, instantaneous protection of the switching devices in extreme load conditions, and transient modes of operation, is not possible. To make matters worse, as the lamp is operated in the steady-state with the circuit in a half-bridge configuration, the dc bus voltage needed is quite high. This contributes to higher switching losses and lower efficiency. Even if zero current switching is employed, the output capacitance devices (Power MOSFETS) contribute to switching losses, and these losses increase at higher bus voltages.
The present invention is directed to a high intensity discharge (HID) lamp driving circuit topology which provides active zero current switching while overcoming the problems associated with the prior art.
According to an object of the present invention, a high intensity discharge (HID) lamp driving circuit is transitionally operable in a lamp starting mode and a lamp running condition. The HID lamp driving circuit comprises a pair of inductor/capacitor filters that are connected to a high intensity discharge lamp in a bridge manner that is alternately operated as one of a ripple reducing filter and a resonant filter; a plurality of switching devices, such as, for example, high frequency switching devices, that are connected with the pair of inductor/capacitor filters; and a power source that provides a voltage to the lamp through the plurality of high frequency switching devices and alternately through one inductor/capacitor filter of the pair of inductor/capacitor filters.
According to an advantage of the present invention, the switching devices may be MOSFET transistors. Each MOSFET transistor may include an integrated high speed diode.
According to another advantage of the present invention, the driving circuit operates in a half-bridge topology during the lamp-starting mode, and a full-bridge topology during the lamp-running mode.
A feature of the present invention is that two switching devices of the plurality of switching devices and one inductor/capacitor filter of the pair of inductor/capacitor filters operate as a high frequency resonant mode switch to turn ON the lamp. The high frequency resonant mode switch is turned ON for a predetermined period of time after a lamp breakdown to provide a smooth transition from when a glowing condition to an arcing condition.
Another feature of the present invention is that one inductor/capacitor filter of the pair of inductor/capacitor filters comprises a high frequency resonant filter during the lamp starting mode, while a remaining inductor/capacitor filter of the pair of inductor/capacitor filters comprises a high frequency ripple reducing filter during the lamp running condition. It is noted that the high frequency resonant filter may comprise one or more (such as, for example, two) capacitors.
A still further advantage of the present invention is that the plurality of switching devices are controlled in an active zero current switching scheme.
According to another object of the present invention, a high intensity discharge (HID) lamp driving circuit, comprises a first pair of switching devices connected to a high frequency resonant filter; a second pair of switching devices connected to a ripple reducing filter; a HID lamp connected between the first pair of switching devices and the second pair of switching devices; a dc power supply connected to the first pair of switching devices and the second pair of switching devices, wherein the first pair of switching devices and the second pair of switching devices are connected to a common ground with the dc power supply. The first switching device operates in a half-bridge topology during a start-up operating mode of the lamp, while the second switching device operates in a full-bridge topology during a steady-state operating mode of the lamp.
According to an advantage of the present invention, the first and second pairs of switching devices comprise high frequency switches, such as, for example, MOSFET transistors.